In recent years, a technique for hot rolling steel sheets has significantly progressed. With this progress, the use environment of hot rolling mill rolls has further become severe. In particular, the production of steel sheets with a large hot rolling load, such as high strength steel sheets and thin-walled products has been increasing recently. Therefore, the improvement in the characteristics of hot rolling mill rolls used, in particular, the improvement in wear resistance has been strongly demanded. To satisfy the demand for improving wear resistance, high-speed steel rolls whose wear resistance has been considerably improved by forming a surface layer having a composition similar to that of high-speed tool steel and dispersing a large amount of hard carbide have been developed and widely used.
Such a high-speed steel roll surface layer material is described in, for example, Patent Literatures 1 and 2. The roll surface layer material described in Patent Literature 1 contains, on a mass % basis, C: 1.5% to 3.5%, Si: 1.5% or less, Mn: 1.2% or less, Ni: 5.5% or less, Cr: 5.5% to 12.0%, Mo: 2.0% to 8.0%, V: 3.0% to 10.0%, and Nb: 0.5% to 7.0%, and Nb and V are contained such that the contents of Nb, V, and C satisfy a particular relationship and the ratio of Nb and V is within a particular range. Thus, even if centrifugal casting is employed, the segregation in the surface layer material is suppressed. This provides excellent wear resistance and crack resistance to a roll surface layer material for a rolling mill.
The roll surface layer material described in Patent Literature 2 contains, on a mass % basis, C: 1.5% to 3.5%, Si: 1.5% or less, Mn: 1.2% or less, Cr: 5.5% to 12.0%, Mo: 2.0% to 8.0%, V: 3.0% to 10.0%, and Nb: 0.5% to 7.0%, and Nb and V are contained such that the contents of Nb, V, and C satisfy a particular relationship and the ratio of Nb and V is within a particular range. Thus, even if centrifugal casting is employed, the segregation in the roll surface layer material is suppressed. This improves the wear resistance and crack resistance, which significantly contributes to an improvement in the productivity in hot rolling.
A roll surface layer material including a roll surface layer with excellent fatigue resistance is described in Patent Literature 3. The roll surface layer material described in Patent Literature 3 contains, on a mass % basis, C: 2.2% to 2.6%, Si: 0.2% to 0.7%, Mn: 0.2% to 0.7%, Cr: 5.0% to 8.0%, Mo: 4.4% to 6.0%, V: 5.3% to 7.0%, and Nb: 0.6% to 1.3%, and the contents of Mo, V, C, and Nb are adjusted such that Mo+V and C−0.24V−0.13Nb are each within a particular range. This can considerably improve the fatigue resistance and suppress the damage to a roll surface. Thus, the lifetime of a roll can be lengthened and the surface quality of rolled products can be considerably improved.